Documentation comment test like Rust

[aprosail]

Clear and concise description of the problem

Rust's documentation comment test, aka doctest, is really good for guaranteeing documentation accuracy and improving code maintainabilities. And I suggest to introduce such features into vitest.

Suggested solution

For example:

/*
 * ```ts
 * import {it, expect} from 'vitest'
 * import {add} from 'xxx'
 * 
 * it('add', () => {
 *   expect(add()).toBe(0)
 *   expect(add(1)).toBe(1)
 *   expect(add(1, 2, 3)).toBe(6)
 * })
 * ```
 */
export function add(...args: number[]) {
  return args.reduce((a, b) => a + b, 0)
}

And when running vitest, it will test all code inside the documentation. In that case, the accuracy of the code example inside documentation comments will be guaranteed, and once reading the doc, people can easily understand how the function works, according to the example code rather than unnecessary descriptions written in natural languages (such as English).

Alternative

No response

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[sheremet-va]

The team already gave our thoughts on this feature here: #3035 (comment)